1. Field of the Invention
The present invention relates to the field of bandgap reference circuits and, more particularly, to start-up techniques for initializing bandgap references.
2. Background of the Related Art
Bandgap reference circuits are utilized to provide precise reference sources. This reference is utilized to generate a reference voltage or current to support other circuitry. One purpose for using the bandgap reference is to provide an accurate and stable output over a temperature range. That is, the bandgap circuit operates with the proper temperature coefficient compensation to correct for variations introduced due to a change in the operating temperature. One area of usage of bandgap references is in the area of analog or mixed signal processing.
A typical bandgap circuit utilizes bipolar transistors to provide the bandgap function. When complementary metal-oxide semiconductor (CMOS) devices are implemented, the bandgap reference generally utilizes parasitic bipolar transistors. The bandgap circuit relies on the difference of the base-emitter junction voltages to provide a linear temperature correction voltage, which is proportional to the absolute temperature (referred to as PTAT). In addition, the base-emitter junction voltage V.sub.BE is proportional to the negative coefficient of temperature. That is, the V.sub.BE measurement is used to track and correct changes in the circuit, caused by a change in temperature. The combination of these two effects results in the bandgap reference responding with a near zero temperature coefficient. Therefore, sources using the bandgap reference respond with a near-zero temperature coefficient at the output.
In a typical CMOS bandgap circuit, a high-gain feedback loop ensures that a correction signal is generated to compensate for the change in the V.sub.BE of the bipolar transistors employed. Generally, the V.sub.BE of individual transistors has a negative coefficient of temperature, while the difference of V.sub.BE of two transistors has a positive coefficient of temperature. Accordingly, as the temperature changes, the change in the V.sub.BE of the transistor(s) in response to the temperature change is sensed to generate a feedback signal to maintain the output of the source relying on the bandgap reference to have substantially constant output (near zero variation of the output).
In order to initialize a bandgap reference (such as at start-up), a start-up circuit is typically utilized. The start-up circuit ensures that the bandgap reference initializes to a desired operating mode, at which time the start-up circuit is disengaged or returned to its idle state. The start-up circuit can be a simple pulse generating circuit, or it can be a circuit which includes components for sensing and turning off the start up signal.
One problem associated with the initializing the bandgap circuit at start-up is in ensuring that the bandgap circuit enters the desired mode of operation. For example, a bandgap circuit could initialize to a zero state where the bipolar transistors are turned-off. Essentially, the zero state is a turned-off state where the circuit is non-operative. Therefore, start-up circuits generally provide some technique to ensure that the start up mode is not the zero-state mode.
In some bandgap reference circuits, there is a possibility that the bandgap circuit may enter into another undesirable mode at start-up. The present invention addresses a concern when some bandgap circuits have a potential of entering into a quasi high-current state, which results in the improper operation of the bandgap reference.